More than 191 tons of construction material had been piled over the weakest areas of an interstate bridge that collapsed last August, according to federal investigators.
In a followup to a preliminary report released in January, the National Transportation Safety Board report that the piles of rock and sand were placed over steel plates that were thinner than they should have been.
Calculations by the agency have determined that the bridge in Minneapolis was carrying a 630-ton load at the time it fell into the Mississippi River. About half of that load was sitting on the center span.
The Aug. 1 collapse killed 13 people and injured 145.
The NTSB had previously cited the too-thin gusset plates and construction project weights as factors in the collapse, but Monday's update included drawings and tables that pinpointed the locations of those heavy loads on the structure.
The agency also said it had gotten "archival information" from an engineering company that may help explain how the bridge ended up with thin gusset plates when it was opened in 1967.
The NTSB had said in January that some of the steel plates were too thin and were fractured. Chairman Mark Rosenker called the plates "the critical factor" in the failure but stopped short of saying they caused the collapse, saying investigators couldn't find the original design calculations.
However, the NTSB now says the Jacobs Engineering Group, which acquired the company that was the original design consultant for the bridge, has provided information "to help investigators better understand what type of system of checks and balances would have been in place when the bridge was designed back in the 1960s." The company acquired by Jacobs was Sverdrup & Parcel.
The NTSB update gave no details on that new information, and NTSB spokesman Terry Williams wouldn't elaborate. Representatives of Jacobs Engineering didn't return a call seeking comment Monday.